1Intravenous administration of paraoxon tLg/kg) to anaesthetized rats induced longlasting, dose-dependent pressor effects. Only after injection of 825 jig/kg paraoxon was the pressor response followed by a depressor effect and a bradycardia that could be blocked by Nmethylatropine. Intracerebroventricular injection of paraoxon into anaesthetized rats also induced pressor effects. 2 In order to elucidate the mechanism of the pressor action rats were given dexetimide, Nmethylatropine, mecamylamine, phentolamine, prazosin, yohimbine, atenolol and metoprolol. If treatment with these drugs resulted in a low initial blood pressure, vasopressin was infused to elevate blood pressure to normal levels. The influence of adrenalectomy, pretreatment with reserpine and midcollicular transection was also examined. 3 The pressor effect of paraoxon was not influenced by N-methylatropine or mecamylamine. However, a combination of these drugs as well as dexetimide, phentolamine or prazosin combined with yohimbine, reduced or prevented the pressor effect. 4 In conscious rats the effects of paraoxon and the action of antimuscarinic drugs upon the pressor response were similar to those observed in anaesthetized animals. 5Acetylcholinesterase activities were measured in various brain regions and in whole blood. Paraoxon concentrations within the CNS were also measured. 6 It is concluded that the pressor effect of paraoxon in anaesthetized and conscious rats is mediated by a central mechanism, although a contribution of peripheral acetylcholinesterase inhibition in sympathetic ganglia to this pressor effect cannot be ruled out.

1 The pressor effects of various tertiary and quaternary isoarecaidine esters in both pithed and anaesthetized rats are reported. In order to elucidate the mechanism of action, various experiments were carried out with the quaternary isoarecaidine methyl ester (Q-4-Me). 2 Treatment with mecamylamine, dexetimide, reserpine and phentolamine abolished or reduced the pressor response to Q-4-Me, whereas pretreatment with cocaine gave rise to a prolonged pressor effect. Plasma noradrenaline levels were significantly increased after the infusion of Q-4-Me. 3 From the results it can be concluded that the pressor effects brought about by the quaternary compounds can be attributed to stimulation of nicotinic and muscarinic receptors in sympathetic ganglia. The consequences of interaction with muscarinic receptors predominate over the effects due to minimal interference with nicotinic receptors by the drug.

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